Chlorofluorocarbon compounds (CFCs) and hydrochlorofluorocarbon compounds (HCFCs) as a class possess unique chemical stability and solvent properties and have until only recently been used in a wide variety of applications, finding utility in drying processes, cleaning processes (e.g., the removal of flux residues from printed circuit boards), and vapor degreasing applications. While these materials were initially believed to be environmentally benign, they now are linked to ozone depletion. According to the Montreal Protocol and its attendant amendments, production and use of CFCs must be discontinued (see, e.g., P.S. Zurer, Looming Ban on Production of CFCs, Halons Spurs Switch to Substitutes, CHEM. & ENG'G NEWS, Nov. 15, 1993, at 12). Characteristics sought in CFC and HCFC replacements, in addition to low ozone depletion potential, typically include low flammability, and low toxicity, and boiling point ranges that are suitable for a variety of solvent cleaning applications. Such replacement solvents also should have the ability to dissolve both hydrocarbon-based and fluorocarbon-based soils.
A group of compounds spotlighted recently as promising substitutes for ozone-depleting solvents are hydrofluoroethers. These compounds, as a class, are particularly promising candidates not only because of their zero ozone-depleting potential, but also because they exhibit very useful solvent properties.
A number of synthetic routes to hydrofluoroethers are known. These methods may be broadly divided into two groups; methods of fluorinating an ether compound, and methods where the ether linkage is formed within a compound by reaction with a fluorine-containing precursor. The former methods include: (1) direct fluorination of an ether compound; and (2) electrochemical fluorination of an ether compound. The latter methods include: (3) the addition reaction of an alcohol to a fluorinated olefin; (4) alkylation of a partially fluorinated alcohol; and (5) non-catalytic alkylation of a fluorinated carbonyl compound with a suitable alkylating agent. Japanese Patent No. JP 6-293686 provides a partial summary description of these varied methods.
Suitable methods for alkylation of fluorinated compounds include those described by French Patent Publication No. 2,287,432 and German Patent Publication No. 1,294,949, and such useful non-catalytic alkylation processes typically comprise the reaction of a perfluorinated acyl fluoride or a perfluorinated ketone with an anhydrous source of fluoride ion (e.g., an alkali metal fluoride such as potassium fluoride, cesium fluoride, or silver fluoride) in an anhydrous polar, aprotic solvent to produce a perfluorinated alkoxide that subsequently is reacted with a suitable alkylating agent such as a dialkyl sulfate, an alkyl halide, or an alkyl perfluoroalkanesulfonate, to produce a primary or secondary hydrofluoroether.
A more recent advance with respect to the above alkylation processes is described by published patent application WO 96/36689 that provides a method of non-catalytic alkylation of fluorinated carbonyl compounds in the presence of a tertiary or aromatic amine.
While the alkylation processes described above may be commercially viable as they are practiced in the art, there is continuing need to improve the economic efficiencies of commercially employed methods of production. In the interests of optimizing the overall process, there also is an ever-present and strong interest in increasing product yields and purities and in decreasing raw material and waste disposal costs. The elimination of potentially toxic alkylating agents such as the commonly used dimethyl sulfate from these reactions also would present significant safety benefits.